1 Technical Field
The present invention relates to rotating electric machines that are used in, for example, motor vehicles as electric motors and electric generators.
2 Description of Related Art
There are known rotating electric machines that are used in motor vehicles as electric motors and electric generators. Those rotating electric machines generally include a rotor and a stator. The stator includes a stator core disposed in radial opposition to the rotor and a stator coil mounted on the stator core. The stator coil is formed by connecting a plurality of electric wires in a predetermined pattern; each of the electric wires has an insulating coat formed on its surface. Moreover, the stator coil has an in-slot part received in slots of the stator core and first and second coil end parts protruding outside of the slots respectively from opposite axial end faces of the stator core. Furthermore, to ensure resistance to vibration, the stator coil is fixed by applying insulating resin, such as a varnish, to the first and second coil end parts.
Moreover, Japanese Patent No. 3144157 discloses a stator for a permanent-magnet electric generator. The stator includes an armature core, a plurality of armature wires mounted on the armature core, and a plurality of bridging wires that connect the armature wires to one another. The stator also includes first, second and third insulating layers. The first insulating layer is formed so as to cover an outer surface of a yoke portion of the armature core. Further, on the first insulating layer, there are arranged the bridging wires. The second insulating layer is formed on the first insulating layer so as to cover the bridging wires. The third insulating layer is formed so as to cover the second insulating layer as well as the first insulating layer. Furthermore, the second insulating layer is formed of either a resin that is not adhesive to both the first and third insulating layers or a resin that has a weak adhesive property to both the first and third insulating layers. Consequently, both the adhesion strength between the first and second insulating layers and the adhesion strength between the second and third insulating layers are low. As a result, when cracks are generated in the first or the third insulating layer, it is possible to prevent application of a large stress to the bridging wires, thereby preventing breakage of the bridging wires.
Furthermore, the inventors of the present invention have found the following problems with the above-described known rotating electric machines.
When a varnish is applied to the first and second coil end parts of the stator coil, the varnish is filled into gaps formed between the electric wires to adhere to the electric wires. Moreover, the varnish has a lower coefficient of linear expansion than the insulating coats of the electric wires forming the stator coil. Consequently, when operation of the rotating electric machine is stopped and thus the ambient temperature of the stator is changed from a high temperature to a low temperature, tensile stress will be induced in the varnish. The tensile stress increases with decrease in the temperature of the varnish, as shown in FIG. 36. When the tensile stress has increased to exceed an allowable stress, cracks will be generated in the varnish. Further, in cases where the adhesion strength between the varnish and the insulating coats of the electric wires is higher than the tensile strength of the insulating coats, when the cracks have advanced to the boundaries between the varnish and the insulating coats, the cracks will cause the Insulating coats to be also cracked together with the varnish, resulting in a puncture or breakdown of the insulating coats. In particular, in cases where the rotating electric machine is configured to cool the stator coil with a coolant, the temperature of the varnish will be decreased more rapidly upon stop of operation of the rotating electric machine, thereby making it easier for a puncture of the insulating coats to occur.